I imagine this gets asked a lot but i'm looking for an in depth explanation since none of the others I've found are detailed enough. I understand that p = mv is not applicable near the speed of light and I think (but am unsure) that the correct equation to use is e = pc. Assuming that e = pc is the correct equation to use then what does 'p' represent? Assuming e = pc is the wrong equation then what is the right one? And can someone (if you can be bothered to waste time on a dumbass like me) explain why p = mv is not applicable? Thanks.